1. Field of the Invention
The present invention relates to equipment for forming an image using an electrophotographic process and, in more particular, to image forming equipment suitable for constructing an electrophotographic process through the use of contact transfer.
2. Related Background Art
Recently, in image forming equipment using an electrophotographic process, instead of corona electrification and corona transfer that have been used conventionally, contact electrification and contact transfer have been studied in order to reduce the amount of ozone generation. As an example of the contact transfer, bias roller transfer has been examined. As a method for realizing the bias roller transfer, there have been studied (1) a constant voltage control method which applies a constant voltage to a transfer member, and (2) a constant current control method which applies a constant current to a transfer member.
However, in the constant voltage control method, since the resistance value of a recording member (such as paper) and a transfer member (such as a transfer roller) vary greatly according to the environment, good transfer has been difficult to attain using a constant voltage. For this reason, an improved constant voltage control method is proposed in U.S. Pat. No. 5,179,397. This proposed method (which is hereinafter referred to as an ATVC control method) detects the resistance value of a roller by applying a constant current to the roller and, in accordance with the detected resistance value, sets up a bias for transfer and then applies a constant voltage to the roller.
On the other hand, a constant current control method for realizing good transfer with respect to variations in the load of a transfer member and a recording member is disclosed in U.S. Pat. No. 3,781,105. However, in the constant current control method, when the width of the recording member varies, poor transfer results. Particularly, when the recording member becomes small, a current flows directly from the transfer member to the surface of an image carrier in an area where the recording member is not present to thereby lower an application voltage. In view of this, an improved method is disclosed in Japanese Patent Publication No. 2-272590 of Heisei which varies a current to be applied to a transfer means according to the width of a recording member.
Also, with respect to the bias roller transfer, the resistance value of the transfer member is also studied in various points along the member. For example, in JAPAN HARD COPY 1991 FALL "Roller transfer method using an elastic member of an intermediate resistance", a relatively high resistance value of the transfer member is used. This requires a high voltage supply source which is capable of outputting a voltage of the order of 4 kV or more, as a transfer supply source. In this case, if a portion of low resistance exists in part in a member of high resistance (which is hereinafter referred to as resistance value variation), or if the equipment is stopped during the paper clogging, then a high voltage of the order of 4 kV can be applied directly to a latent image carrier to open up a hole in a photosensitive layer on the latent image carrier. This in turn results in electrification and poor transfer (which is hereinafter referred to as a pin hole). The pin hole is found especially when an organic photosensitive member having a low dielectric strength is used as the latent image carrier. In order to prevent such a pin hole, there is also proposed a structure in which a high resistance layer is coated on the outer layer of the transfer member (transfer roller) to thereby produce a multi-layer roller. If a transfer member of low resistance is used, then a small bias is required for transfer even if the resistance value variation exists and thus use of the transfer member of low resistance is advantageous with respect to the pin hole. However, conventionally, it has been considered impossible to put this into practical use, because, if a transfer member of low resistance (5.times.10.sup.8 .OMEGA. or less) is used, then the surface potential of the latent image carrier is turned into a reversed polarity due to the action of the transfer bias so that a ghost phenomenon will occur at the cycle of the latent image carrier. (This phenomenon is hereinafter referred to as an image memory, or, a ghost phenomenon.)
And, toner used in the contact transfer is also under study. For example, although not directly connected with the contact transfer, as not only an improvement in the deteriorated toner but also an improvement in a developing method, there is proposed a developing method which adds and mixes externally two kinds of fine powder having different mean particle diameters from each other, as can be seen in Japanese Patent Publication No. 2-45188 of Heisei.
However, the above-mentioned conventional techniques have the following problems to be solved.
First, in the ATVC control as disclosed in U.S. Pat. No. 5,179,397 or such variable current control as disclosed in Japanese Patent Publication No. 2-272590 of Heisei, means used to detect the resistance value of the transfer member, the width of the recording member and the like are necessary. Further, of course, a control system must be set up which uses such means. For this reason, these control methods are very disadvantageous in the cost and installation space of image forming equipment. Also, an expensive and complicated supply source is required in order to process the signal of the detect means by use of a microprocessor and to determine and change the output of a high voltage supply source.
Second, since the multi-layer roller used as the pin hole preventive means is a complex roller, rather than a single layer roller, it is overwhelmingly disadvantageous in the manufacturing method, manufacturing time, cost, and handling.
Thirdly, it has been found that when a toner composed of resin particles with two or more kinds of external additives having different particle diameters is used in a contact transfer device, poor transfer can occur. Examples of poor transfer are void or hollow character phenomenon (the phenomenon in which the central portion of a character is not transferred to the recording member, hereinafter referred to as a white void), density reduction contamination of the backside of the recording member due to fogging, and other unfavorable phenomena.